Nitride semiconductors have properties, such as high saturation electron velocity and wide band gaps. Thus, attempts have been made to use nitride semiconductors for high-power semiconductor devices with high-breakdown voltages. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV, which is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV). Hence, GaN has a high breakdown field strength. Accordingly, GaN is a highly promising material for high-power semiconductor devices, which operate at high voltages, used in power supplies.
Many reports have been made of field-effect transistors, in particular, high electron mobility transistors (HEMTs) as semiconductor devices composed of nitride semiconductors. For example, with respect to GaN-based HEMTs (GaN-HEMTs), AlGaN/GaN HEMTs including electron transit layers composed of GaN and electron supply layers composed of AlGaN have been receiving attention. In an AlGaN/GaN HEMT, a difference in lattice constant between GaN and AlGaN causes strain in AlGaN. The strain-induced piezoelectric polarization and the spontaneous polarization of AlGaN results in a high-density two-dimensional electron gas (2DEG). Thus, AlGaN/GaN HEMTs are promising high-breakdown voltage devices usable for, for example, high-efficiency switch elements and electric vehicles.
Japanese Laid-open Patent Publication Nos. 2012-9630, 2010-239034, and 2007-251144 are reference documents cited in this specification.
To produce a high-quality nitride semiconductor device at low cost, for example, a nitride semiconductor layer may be formed on a common Si substrate by an epitaxial growth method.
However, Si and a nitride semiconductor differ significantly from each other not only in lattice constant but also in thermal expansion coefficient. Thus, in order to grow a high-quality nitride semiconductor layer, an appropriately designed buffer layer is used.
To resolve the lattice mismatch and the difference in thermal expansion coefficient between Si and the nitride semiconductor as much as possible, the buffer layer is formed so as to have a relatively large thickness.
As a result, in the case where a HEMT including a nitride semiconductor multilayer structure containing the buffer layer is fabricated, there is a problem in which a leakage current (off-leakage current) passes through the buffer layer or a lower layer region of an electron transit layer. In this case, it is difficult to control the residual carrier concentration, so that it is very difficult to inhibit the occurrence of the off-leakage current, thereby leading to a reduction in breakdown voltage.